Method of driving X-ray imaging device

Details Method of driving X-ray imaging device Method of driving X-ray imaging device

2000-12-28

2002-09-24

Porta, David P. (Department: 2882)

X-ray or gamma ray systems or devices

Electronic circuit

With display or signaling

C378S091000, C378S207000, C250S370090

Reexamination Certificate

active

06456689

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 1999-68051, filed on Dec. 31, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving an X-ray imaging device, and more particularly to a method of driving an X-ray imaging device that is capable of improving a picture's contrast ratio and of shortening a driving time.
2. Discussion of the Related Art
Imaging systems that employ X-rays have been used in numerous medical, scientific and industrial applications. Such imaging systems include X-ray imaging devices. One type of X-ray imaging device uses an array of photosensitive cells on an array panel to sense the intensity of X-rays passing through an object. Those X-ray intensities are used to produce an image of the object. In operation, the photosensitive cells generate electric charges in proportion to the intensity of the X-rays. The electric charges from the photosensitive cells are applied to a signal converter that converts the charges into electrical signals, which are in turn sent to an image output device. The image output device processes the electrical signals so as to display the X-ray intensity pattern on a screen.
FIG.
1
A and
FIG. 1B
respectively illustrate a sectional schematic view and a planar schematic view of a photosensitive cell in a photosensitive cell array panel. Referring to
FIG. 1A
, the photo-sensitive cell includes a gate line
22
, a thin film transistor (TFT)
24
, a charging capacitor (Cst) layer that are formed on a glass substrate
20
, and a pixel electrode
32
that is connected to a drain electrode
26
of the TFT and to the charging capacitor Cst. The photo-sensitive cell further includes a gate electrode
30
, a source electrode
28
, a photo-sensing layer
34
on the pixel electrode
32
, an insulating layer
36
on the photo-sensing layer
34
, and an upper electrode
38
.
The photo-sensing layer
34
is a photoconductive layer that is used for sensing X-rays and for converting those rays to electric charges. The photo-sensing layer
34
is beneficially formed from amorphous selenium having a thickness of hundreds of &mgr;m.
As shown in FIG.
1
A and
FIG. 1B
, the gate electrode
30
electrically connects to the gate line
22
. Control signals are applied to the TFT by the gate line and by the gate electrode. The source electrode
28
connects to a data line
40
(see FIG.
1
B). Beneficially, the data line
40
is perpendicular to the gate line
22
. As previously indicated, the drain electrode
26
of the TFT
24
connects to the pixel electrode
32
. As indicated by
FIG. 1B
, the pixel electrode
32
has an area that is as large as possible. This aids the collection of electric charges from the photo-sensing layer
34
. The collected charges are then stored in the charging capacitor Cst. A high voltage generator
42
connects to the upper electrode
38
. That voltage generator supplies a voltage that generates an electric field through the photo-sensing layer
34
.
As X-rays irradiate an object, the rays that pass through the object are incident on the photo-sensing layer
34
. Those incident X-rays produce electron-hole pairs in the photo-sensing layer
34
. When a high voltage (several kilovolts) from the high voltage generator
42
is applied to the upper electrode
38
, the electron-hole pairs within the photo-sensing layer
34
are separated by the resulting electric field. As shown in
FIG. 1A
, the holes are collected by the pixel electrode
32
and are stored in the charging capacitor Cst. Alternatively, electrons can be collected and stored. The TFT
24
acts as a switch that controls the discharge of electric charges in the charging capacitor Cst. When a gate voltage is applied to the gate electrode
30
via the gate line
22
, a channel is defined between the source electrode
28
and the drain electrode
26
. At this time, the electric charge in the charging capacitor Cst is applied to the source electrode
28
via the drain electrode
26
. The electric charges applied to the source electrode
28
are then output over the data line
40
, which is connected to the source electrode
28
.
FIG. 2
illustrates an X-ray imaging system having a driving apparatus that converts electric charges stored in a photo sensitive cell array panel into electrical signals that can be output as an image. The X-ray imaging system includes a photo sensitive cell array panel
60
having a plurality of photo-sensing cells
62
that are arranged in a matrix. A gate driver
64
connects to gate lines, that gate lines GL
1
to GLm, that are provided on the photo sensitive cell array panel
60
. A data reader
66
connects to data lines, the data lines DL
1
to DLn, that are also provided on the photo sensitive cell array panel
60
. An output
68
displays the electrical signals from the data reader
66
as an image.
In the photo sensitive cell array panel
60
the photo-sensing cells
62
are individually positioned at intersections between the gate lines GL
1
to GLm and the data lines DL
1
to DLn. In
FIG. 2
, each of the photo-sensing cells
62
consists of a photo sensor
70
, a charging capacitor Cst and a TFT
72
. For each photo-sensing cell
62
, a gate electrode
74
connects to the gate driver
64
by one of the gate lines GL
1
to GLm, and a source electrode
76
connects to the data reader
66
by one of the data lines DL
1
to DLn. Furthermore, a drain electrode
78
connects to a charging capacitor Cst.
When a gate control signal from the gate driver
64
is applied, via one of the gate lines GL
1
to GLm, to the gate electrode
74
of the TFT
72
of a photo-sensing cell
62
, a conductive channel is defined between the drain electrode
78
and the source electrode
76
of the TFT
72
. Electric charges stored in the charging capacitor Cst are then transferred to the data reader
66
, via one of the data lines DL
1
to DLn, by the source electrode
76
.
The gate driver
64
sequentially applies pulse-shaped gate control signals to the gate lines GL
1
to GLm on the photo sensitive cell array panel
60
. When a gate control signal is applied to one of the gate lines, the electric charges stored in the photo-sensing cells
62
connected to that gate line are applied to the data reader
66
, thereby forming a scan line. The data reader
66
typically includes n charge amplifiers (not shown) connected to the n data lines DL
1
to DLn. The charge amplifiers convert the flow of electric charges (current signals) from the data lines DL
1
to DLn into voltage signals. Thus, the data reader
66
generates electrical data signals that correspond to electric charges from the photo sensitive cell array panel
60
.
The data reader
66
sequentially applies the n electrical data signals, each of which depends on the intensity of the X-ray energy irradiated onto a photo sensitive cell, and a reference signal to the output
68
. The output
68
includes a differential amplifier and an analog-to-digital converter (which are not shown). The electrical data signals input to the output
68
is an analog signal that includes noise. The output
68
differentially amplifies each electrical data signal and the reference signal to remove that noise, and then converts the noise-removed analog signal into a digital signal that is suitable for display on a screen as part of an image.
In an X-ray imaging device that operates as described above, the period of time that the high voltage is applied by the high voltage generator
42
to the upper electrode
38
, and the period of time that X-rays are irradiated have a significant impact on the quality of the output image. An instantaneous current is generated at the photo-sensing layer
34
when the high voltage is first applied to the upper electrode
38
. This accumulates a dark charge in the charging capacitor Cst. When the high voltage is removed, a variation in the charge stored in the charging capacitor Cst occurs, and thus a signal va

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